Enormous efforts have been recently attracted to seek new materials and/or novel structures for efficient solar energy conversions owing to the increasing awareness of devastating environmental impact of fossil fuel usages in meeting energy needs. To be economically competitive, solar energy needs to be converted into other forms that can be directly utilized with high efficiency and low cost. Among the materials that have been studied, titanium dioxide (TiO2) is particularly appealing, owing to the fact that TiO2 has a high incident photon-to-electrical charge conversion efficiency (IPCE), and remarkable chemical stability. Both photovoltaic and photoelectrochemical cells (PECs) based on TiO2 have been investigated in recent years. Although different in the forms of conversion products, these processes share the same scientific concepts. In brief, photons are absorbed to separate charges, which are then either directly collected (for photovoltaics) or utilized to perform chemical reactions (for solar fuels).
The implementations of TiO2 in solar conversions remain limited. The performance of TiO2-based devices is compromised by several fundamental deficiencies of TiO2, including: low conductivity, poor absorption in the visible range of the solar spectrum, and the lack of matching p-type materials.